Heretofore, proximity detecting apparatus have been composed of active elements that require a continuous supply of energy.
One such apparatus has a continuously operating oscillator connected to a sensing element. When the capacitance on the sensing element changed due to the presence of an intruder in the field of the sensing element, the capacitance reactance would disturb the tuning of the oscillator circuit and it would cease oscillating to produce an increased current flow in the oscillator to activate a relay in an alarm circuit.
Another apparatus has more than one oscillator, one tuned to operate at a constant frequency and another operable at a frequency determined by the charge and voltage on the sensing element. When there is a change in the charge and thus the voltage at the sensing element a beat frequency would be produced which would be detected to cause an alarm to be activated.
Still another type of apparatus utilizes a metal oxide silicon field effect transistor instead of a vacuum-tube oscillator and the usual gas-type relay tube has been replaced with an npn bipolar transistor. When energized the oscillator will oscillate if there is no intrusion of the sensing element field. When intrusion occurs the oscillation ceases and there will be an increase in current in the transistor. This energizes the npn that is in the circuit of a relay that controls the alarm circuit.
There are several drawbacks to such apparatus. The first is that they all use oscillators which must be kept oscillating during the quiescent period between intrusions and this represents a constant expenditure of electrical energy. This in turn makes it necessary to utilize the power lines as a source of energy with the attending drawbacks of power lines for power failure, providing easy access to circuits that might be cut and the attending transient peak voltages that often trigger false alarms.
With the advent of the microelectronics and integrated circuit chips it is now possible to procure the extreme sensitivity, lower power drain and greater reliability. It is possible to provide circuits that have practically zero current drain during the quiescent periods thus making it possible to utilize batteries as a source of power and which now can serve for long periods of time without replacement. It is now possible to produce detecting apparatus smaller in size, with a minimum of complexity, less costly to produce and operate and easy to use. With these possibilities in mind it is the intent to provide a much improved proximity detector apparatus that may be used with simple and complex security systems.